Support strict mode for JsonParser

gson/src/main/java/com/google/gson/Gson.java

@@ -930,7 +930,8 @@ public void toJson(Object src, Type typeOfSrc, JsonWriter writer) throws JsonIOE
     Strictness oldStrictness = writer.getStrictness();
     if (this.strictness != null) {
       writer.setStrictness(this.strictness);
-    } else if (writer.getStrictness() != Strictness.STRICT) {
+    } else if (writer.getStrictness() == Strictness.LEGACY_STRICT) {
+      // For backward compatibility change to LENIENT if writer has default strictness LEGACY_STRICT
       writer.setStrictness(Strictness.LENIENT);
     }
 
@@ -1013,7 +1014,8 @@ public void toJson(JsonElement jsonElement, JsonWriter writer) throws JsonIOExce
 
     if (this.strictness != null) {
       writer.setStrictness(this.strictness);
-    } else if (writer.getStrictness() != Strictness.STRICT) {
+    } else if (writer.getStrictness() == Strictness.LEGACY_STRICT) {
+      // For backward compatibility change to LENIENT if writer has default strictness LEGACY_STRICT
       writer.setStrictness(Strictness.LENIENT);
     }
 
@@ -1347,7 +1349,8 @@ public <T> T fromJson(JsonReader reader, TypeToken<T> typeOfT)
 
     if (this.strictness != null) {
       reader.setStrictness(this.strictness);
-    } else if (reader.getStrictness() != Strictness.STRICT) {
+    } else if (reader.getStrictness() == Strictness.LEGACY_STRICT) {
+      // For backward compatibility change to LENIENT if reader has default strictness LEGACY_STRICT
       reader.setStrictness(Strictness.LENIENT);
     }
 

gson/src/main/java/com/google/gson/JsonParser.java

@@ -87,9 +87,10 @@ public static JsonElement parseReader(Reader reader) throws JsonIOException, Jso
    * methods, no exception is thrown if the JSON data has multiple top-level JSON elements, or if
    * there is trailing data.
    *
-   * <p>The JSON data is parsed in {@linkplain JsonReader#setStrictness(Strictness) lenient mode},
-   * regardless of the strictness setting of the provided reader. The strictness setting of the
-   * reader is restored once this method returns.
+   * <p>If the {@linkplain JsonReader#getStrictness() strictness of the reader} is {@link
+   * Strictness#STRICT}, that strictness will be used for parsing. Otherwise the strictness will be
+   * temporarily changed to {@link Strictness#LENIENT} and will be restored once this method
+   * returns.
    *
    * @throws JsonParseException if there is an IOException or if the specified text is not valid
    *     JSON
@@ -98,7 +99,10 @@ public static JsonElement parseReader(Reader reader) throws JsonIOException, Jso
   public static JsonElement parseReader(JsonReader reader)
       throws JsonIOException, JsonSyntaxException {
     Strictness strictness = reader.getStrictness();
-    reader.setStrictness(Strictness.LENIENT);
+    if (strictness == Strictness.LEGACY_STRICT) {
+      // For backward compatibility change to LENIENT if reader has default strictness LEGACY_STRICT
+      reader.setStrictness(Strictness.LENIENT);
+    }
     try {
       return Streams.parse(reader);
     } catch (StackOverflowError e) {

gson/src/main/java/com/google/gson/stream/JsonReader.java

@@ -336,7 +336,7 @@ public final void setLenient(boolean lenient) {
   /**
    * Returns true if the {@link Strictness} of this reader is equal to {@link Strictness#LENIENT}.
    *
-   * @see #setStrictness(Strictness)
+   * @see #getStrictness()
    */
   public final boolean isLenient() {
     return strictness == Strictness.LENIENT;
@@ -393,6 +393,7 @@ public final boolean isLenient() {
    * </dl>
    *
    * @param strictness the new strictness value of this reader. May not be {@code null}.
+   * @see #getStrictness()
    * @since $next-version$
    */
   public final void setStrictness(Strictness strictness) {

gson/src/main/java/com/google/gson/stream/JsonWriter.java

@@ -316,7 +316,7 @@ public final void setLenient(boolean lenient) {
   /**
    * Returns true if the {@link Strictness} of this writer is equal to {@link Strictness#LENIENT}.
    *
-   * @see JsonWriter#setStrictness(Strictness)
+   * @see #getStrictness()
    */
   public boolean isLenient() {
     return strictness == Strictness.LENIENT;
@@ -338,6 +338,7 @@ public boolean isLenient() {
    * </dl>
    *
    * @param strictness the new strictness of this writer. May not be {@code null}.
+   * @see #getStrictness()
    * @since $next-version$
    */
   public final void setStrictness(Strictness strictness) {

gson/src/test/java/com/google/gson/JsonParserTest.java

@@ -173,15 +173,30 @@ public void testReadWriteTwoObjects() throws Exception {
     assertThat(actualTwo.stringValue).isEqualTo("two");
   }
 
+  @Test
+  public void testLegacyStrict() {
+    JsonReader reader = new JsonReader(new StringReader("unquoted"));
+    Strictness strictness = Strictness.LEGACY_STRICT;
+    // LEGACY_STRICT is ignored by JsonParser later; parses in lenient mode instead
+    reader.setStrictness(strictness);
+
+    assertThat(JsonParser.parseReader(reader)).isEqualTo(new JsonPrimitive("unquoted"));
+    // Original strictness was restored
+    assertThat(reader.getStrictness()).isEqualTo(strictness);
+  }
+
   @Test
   public void testStrict() {
     JsonReader reader = new JsonReader(new StringReader("faLsE"));
     Strictness strictness = Strictness.STRICT;
-    // Strictness is ignored by JsonParser later; always parses in lenient mode
     reader.setStrictness(strictness);
 
-    assertThat(JsonParser.parseReader(reader)).isEqualTo(new JsonPrimitive(false));
-    // Original strictness was restored
+    var e = assertThrows(JsonSyntaxException.class, () -> JsonParser.parseReader(reader));
+    assertThat(e)
+        .hasCauseThat()
+        .hasMessageThat()
+        .startsWith("Use JsonReader.setStrictness(Strictness.LENIENT) to accept malformed JSON");
+    // Original strictness was kept
     assertThat(reader.getStrictness()).isEqualTo(strictness);
   }
 }

gson/src/test/java/com/google/gson/stream/JsonReaderTest.java

@@ -41,6 +41,12 @@
 @SuppressWarnings("resource")
 public final class JsonReaderTest {
 
+  @Test
+  public void testDefaultStrictness() {
+    JsonReader reader = new JsonReader(reader("{}"));
+    assertThat(reader.getStrictness()).isEqualTo(Strictness.LEGACY_STRICT);
+  }
+
   @SuppressWarnings("deprecation") // for JsonReader.setLenient
   @Test
   public void testSetLenientTrue() {